Bones Organs that contain several types of tissues Dominated by bone connective tissue Contain nervous tissue and blood tissue Contain cartilage in articular cartilages Contain epithelial tissue lining blood vessels

Function of Bones Support – provides hard framework Movement – skeletal muscles use bones as levers Protection of underlying organs Mineral storage – reservoir for important minerals Blood-cell formation – bone contains red marrow

Classification of Bones Figure 6.1

Classification of Bones Long bones – longer than wide – a shaft plus ends Short bones – roughly cube-shaped Flat bones – thin and flattened, usually curved Irregular bones – various shapes, do not fit into other categories

Bone Surface Markings Depressions or Openings Foramen – opening Fissure – narrow slits between bones Fossa – shallow depression Sulcus – groove Meatus – tubelike passageway or canal

Bone Surface Markings Processes or outgrowths (joints or attachments) Condyle – large round protuberance Facet – smooth flat articular surface (see vertebrae) Trochanter – very large projection Tuberosity – large rounded roughened projection

Gross Anatomy of Bones Compact bone – dense outer layer of bone Spongy bone – internal network of bone

Structure of a Typical Long Bone Diaphysis – “shaft” of a bone Epiphysis – ends of a bone Blood vessels – well vascularized Medullary cavity – hollow cavity – filled with marrow Membranes – periosteum, Sharpey’s fibers, and endosteum

Structure of a Long Bone Figure 6.3a-c

Structure of Short, Irregular, and Flat Bones

Figure 6.5a Gross Anatomy of Bones Bone design and stress Anatomy of a bone reflects stresses Compression and tension greatest at external surfaces

Microscopic Structure of Compact Bones Figure 6.6

Chemical Composition of Bone 35% organic components Composed of cells, fibers, and organic substances Collagen – abundant 65% inorganic mineral salts Primarily calcium phosphate Resists compression

Bone Development Ossification (osteogenesis) – bone-tissue formation Membrane bones – formed directly from mesenchyme (bones of the skull and clavicles) Intramembranous ossification Majority of the bones – develop initially from hyaline cartilage Endochondral ossification

Intramembranous Ossification Figure 6.9 (1), (2)

Intramembranous Ossification Figure 6.9 (3), (4)

Endochondral Ossification All bones except some bones of the skull and clavicles Bones are modeled in hyaline cartilage Begins forming late in 2nd month of human development Continues forming until early adulthood

Stages in Endochondral Ossification Figure 6.10

Anatomy of Epiphyseal Growth Areas In epiphyseal plates of growing bones Cartilage is organized for quick, efficient growth Cartilage cells form tall stacks Chondroblasts at the top of stacks divide quickly Pushes the epiphysis away from the diaphysis Lengthens entire long bone

Structure of a Long Bone Figure 6.3a-c

Osteoclast – A Bone-Degrading Cell A giant cell with many nuclei Crawls along bone surfaces Breaks down bone tissue Secretes concentrated hydrochloric acid Lysosomal enzymes are released Figure 6.13a

Anatomy of Epiphyseal Growth Areas Older chondrocytes signal surrounding matrix to calcify Older chondrocytes then die and disintegrate Leaves long trabeculae (spicules) of calcified cartilage on diaphysis side Trabeculae are partly eroded by osteoclasts Osteoblasts then cover trabeculae with bone tissue Trabeculae finally eaten away from their tips by osteoclasts

Postnatal Growth of Endochondral Bones During childhood and adolescence Bones lengthen entirely by growth of the epiphyseal plates Cartilage is replaced with bone tissue as quickly as it grows Epiphyseal plate maintains constant thickness Whole bone lengthens

Postnatal Growth of Endochondral Bones As adolescence draws to an end Chondroblasts divide less often Epiphyseal plates become thinner Cartilage stops growing Replaced by bone tissue Long bones stop lengthening when diaphysis and epiphysis fuse

Postnatal Growth of Endochondral Bones Growing bones widen as they lengthen Osteoblasts – add bone tissue to the external surface of the diaphysis Osteoclasts – remove bone from the internal surface of the diaphysis Appositional growth – growth of a bone by addition of bone tissue to its surface

Hormonal Regulation of Bone Growth Growth hormone – produced by the pituitary gland Stimulates epiphyseal plates Thyroid hormone – ensures that the skeleton retains proper proportions Sex hormones Promote bone growth Later induces closure of epiphyseal plates

Bone Remodeling Bone deposit and removal Occurs at periosteal and endosteal surfaces Bone remodeling Bone deposition – accomplished by osteoblasts Bone reabsorption – accomplished by osteoclasts

Stages of Healing a Fracture Figure 6.14

Common Types of Fractures Table 6.1

Common Types of Fractures Table 6.1

Common Types of Fractures Table 6.1

Disorders of Bones Osteoporosis – characterized by low bone mass Bone reabsorption outpaces bone deposition Occurs most of in women after menopause

Osteoporosis Figure 6.15

Disorders of Bones Osteomalacia – occurs in adults – bones are inadequately mineralized Rickets – occurs in children – analogous to osteomalacia Paget's disease – characterized by excessive rate of bone deposition Osteosarcoma – a form of bone cancer

The Skeleton Throughout Life Cartilage grows quickly in youth Skeleton shows fewer chondrocytes in the elderly Bones are a timetable Mesoderm – gives rise to embryonic mesenchyme cells Mesenchyme – produces membranes and cartilage Membranes and cartilage ossify

The Skeleton Throughout Life Skeleton grows until the age of 18–21 years In children and adolescents Bone formation exceeds rate of bone reabsorption In young adults Bone formation and bone reabsorption are in balance In old age reabsorption predominates Bone mass declines with age